This invention relates generally to semiconductor devices, and more particularly the invention relates to high frequency (RF and microwave) power transistors and the improvement of breakdown voltage of such devices.
Power transistors and diodes are typically fabricated in a device region in a semiconductor substrate with the device region surrounded by a thick field oxide which functions as a junction termination. See FIG. 1 which illustrates in section a base region 10 of a bipolar transistor in a silicon substrate 12 and surrounded by field oxide 14. The breakdown voltage (BV) is limited by peripheral effects including curvature of the PN junction separating the base and collector due to dopant suck-up (boron for an NPN transistor base) in the field oxide and diffusion into the silicon. The oxide/silicon interface charge typically results in reduced breakdown for the NPN transistor.
FIG. 2 illustrates a modification of the structure in FIG. 1 in which a deep extrinsic base extension 11 is formed to provide a radius of curvature sufficient to maintain the required collector-base junction. Formation of the deep extrinsic base extension does require additional processing in masking, dopant implanting, and drive-in. Further, the structure still remains sensitive to oxide/silicon fixed charge and boron suck-up into the field oxide (NPN transistor). Additionally, higher device capacitance results due to increased depth and lateral dimensions of the base region.
FIG. 3 illustrates another modification which employs a lightly doped junction extension 11 around the perimeter. Again, the junction extension requires additional processing including masking, implant, and high temperature drive-in. The increased junction area results in higher device capacitance due to the increased lateral dimensions of the collector base junction.
FIG. 4 illustrates in section a fully oxide walled junction transistor which has increased breakdown voltage due to the improved electric field distribution therein. Device capacitance is minimized due to no lateral encroachment in the collector base PN junction. However, again complex processing requiring chemical-mechanical polishing, deposited conformal oxides, isotropic etching, and planarization are required.
FIG. 5 illustrates in section another prior art structure which utilizes a field plate 18 overlying the field oxide 14 and positioned around the device region in contact with the base 10. The field plate is capacitively coupled to the underlying semiconductor wafer 12 and functions as a capacitor between the base and collector of the transistor. The capacitor tends to maintain the PN junction between the base and collector by repelling electrons from the collector region (for an NPN transistor). However, the thickness of the field oxide limits the capacitance and the effect thereof in increasing breakdown voltage.
Accordingly, there is a need for a simple, low cost method and structure for increasing the junction breakdown voltage of a semiconductor device, especially for an RF and microwave power transistor, which minimizes any increase in junction capacitance.